Various aspects described herein relate to satellite communication, and more particularly but not exclusively, to handoff for non-geosynchronous satellite communication.
Satellite-based communication systems may include gateways and one or more satellites to relay communication signals between the gateways and one or more user terminals. A gateway is an earth station having an antenna for transmitting signals to, and receiving signals from, communication satellites. A gateway provides communication links, using satellites, for connecting a user terminal to other user terminals or users of other communication systems, such as a public switched telephone network, the Internet and various public and/or private networks. A satellite is an orbiting receiver and repeater used to relay information.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the “footprint” of the satellite. The footprint of a satellite is the geographic region on the surface of the Earth within the range of signals of the satellite. The footprint is usually geographically divided into “cells” (e.g., “beams”), through the use of beam-forming antennas. Each cell (e.g., beam) covers a particular geographic region within the footprint. Cells from the same satellite or from different satellites may overlap (e.g., partially overlap). For example, the beams of a particular satellite may be directed so that more than one beam from that satellite covers the same specific geographic region.
Geosynchronous satellites have long been used for communication. A geosynchronous satellite is stationary relative to a given location on the Earth, and thus there is little timing shift and Doppler frequency shift in radio signal propagation between a communication transceiver on the Earth and the geosynchronous satellite. However, because geosynchronous satellites are limited to a geosynchronous orbit (GSO), which is a circle having a radius of approximately 42,164 km from the center of the Earth directly above the Earth's equator, the number of satellites that may be placed in the GSO is limited.
As alternatives to geosynchronous satellites, communication systems which utilize a constellation of satellites in non-geosynchronous orbits, such as low-earth orbits (LEO), have been devised to provide communication coverage to the entire Earth or at least large parts of the Earth. In non-geosynchronous satellite-based systems, such as LEO satellite-based systems, the satellites move relative to ground-based communication devices such as gateways or user terminals. Thus, at some point in time, a user terminal will be handed-off from one satellite to another.